1. Field of the Invention
The present invention relates to a blood pressure measuring apparatus having a function of detecting a pulse wave to be analyzed.
2. Related Art Statement
A pulse wave produced from an artery of a living subject, i.e., an arterial pulse wave provides a plurality of sorts of information on the physical condition of the subject. In view of this, the pulse wave is often analyzed for diagnosing the physical condition of the subject. For instance, an augmentation index (AI) is determined or obtained based on the arterial pulse wave, for evaluating arteriosclerosis.
As described in xe2x80x9cPulse-Wave Velocityxe2x80x9d pages 18, 19, 22, 1st edition, May 1, 2002, by Toshio OZAWA and Yoshiaki MASUDA, published by MEDICAL VIEW, the augmentation index is calculated as a proportion of a difference between a height (magnitude) of a tidal wave, and a height (magnitude) of a percussion wave, of a heartbeat-synchronous pulse of the arterial pulse wave, to a height (magnitude) of a maximum point of the heartbeat-synchronous pulse of the arterial pulse wave. Since, as arteriosclerosis advances, a reflected-wave component of the pulse wave returns back faster than normal, the magnitude of the tidal wave becomes greater than that of the percussion wave, and the augmentation index increases.
The arterial pulse wave may be invasively obtained by inserting a pressure-sensing element or a catheter directly in the artery of the subject. Clinically, there is used a pressure pulse wave which is obtained by using a pulse wave detector, because this method is easier for obtaining the pulse wave than the invasive method. As the pulse wave detector, there is often used a multi-sensor-type pulse wave detector (available from Colin Corporation, Japan) as described in the above-indicated document. The multi-sensor-type pulse wave detector has about thirty sensors, and is arranged to record automatically the optimum one of respective waveforms detected by the thirty sensors, the optimum waveform having the greatest SIN ratio. The thus arranged multi-sensor-type pulse wave detector is convenient for detecting the pulse wave since the detector can be used without skill. The multi-sensor-type pulse wave detector is disclosed in JP-A-2001-190509, for instance. The disclosed multi-sensor-type pulse wave detector employs a so-called xe2x80x9ctonometric methodxe2x80x9d wherein a pressure pulse wave sensor is adapted to be pressed against an artery of a subject such that a portion of the wall of the artery is substantially flattened, for detecting a pressure pulse wave.
The above-described multi-sensor-type pulse wave detector as the tonometric-type sensor is expensive, and needs cumbersome operation for detecting the pressure pulse wave. Accordingly, there has been a need to utilize and analyze, for making a diagnosis on a patient, a cuff pulse wave obtained from a cuff by using a cuff-equipped blood pressure measuring apparatus which is widely used since it is inexpensive and simple to handle. However, as the cuff pulse wave is a volumetric pulse wave representing a volume of blood, the waveform of the cuff pulse wave is not completely identical with that of the pressure pulse wave detected through the above-described pulse wave detector. Accordingly, the knowledge which has conventionally been obtained by analyzing the pressure pulse wave detected through the pulse wave detector cannot be readily utilized in diagnosing a patient based on a cuff pulse wave.
It is therefore an object of the present invention to provide a blood pressure measuring apparatus with a pulse-wave detecting function, the apparatus being capable of converting a waveform of a cuff pulse wave into a waveform of a pressure pulse wave.
As a result of an extensive study made by the inventor, it has been found that the difference between the waveform of the cuff pulse wave and the waveform of the pressure pulse wave increases in a direction toward the maximum or peak point of the cuff pulse wave, and that the amount of difference between the waveform of the cuff pulse wave and the waveform of the pressure pulse wave depends on a proportion of a difference of a height or magnitude of each point of the waveform of the cuff pulse wave from a height or magnitude of a minimum point of the waveform, to a pulse pressure as the difference of a magnitude of a maximum point from the magnitude of the minimum point of the pulse. The present invention has been developed based on these findings.
The above-indicated object of the present invention has been achieved by the present invention. According to the present invention, there is provided a blood pressure measuring apparatus, comprising a cuff which is adapted to be worn on a body portion of a living subject, and determining a blood pressure value of the subject based on a signal which is obtained from the cuff when a pressure in the cuff is changed, wherein the apparatus further comprises: a cuff pulse wave detecting device which detects a cuff pulse wave as a pulse wave which is produced from an artery of the subject and is transmitted to the cuff, the cuff pulse wave comprising at least one heartbeat-synchronous pulse which is produced from the artery in synchronism with at least one heartbeat of the subject; a converting means for converting, based on the blood pressure value determined by the apparatus, a magnitude of each point of the heartbeat-synchronous pulse of the cuff pulse wave, into a pressure value; and a pseudo-pressure-pulse-wave determining means for determining a pseudo pressure pulse wave by correcting the respective pressure values of the respective points of the heartbeat-synchronous pulse, provided by the converting means, according to at least one correction relationship between (A) proportion of difference of pressure value of each point of heartbeat-synchronous pulse from pressure value of minimum point of the pulse, to pulse pressure as difference of pressure value of maximum point of the pulse from the pressure value of the minimum point of the pulse, and (B) correction amount, in which greater correction amount corresponds to greater proportion.
In the present blood pressure measuring apparatus, the converting means converts, based on the blood pressure value determined by the blood pressure measuring apparatus, a magnitude of each point of the heartbeat-synchronous pulse of the cuff pulse wave detected by the cuff pulse wave detecting device, into a pressure value. Subsequently, the pseudo-pressure-pulse-wave determining means determines the pseudo pressure pulse wave by correcting the respective pressure values of the respective points of the heartbeat-synchronous pulse. Therefore, the determined pseudo pressure pulse wave can be effectively utilized for making a diagnosis on the patient, without a need of detecting a pressure pulse wave by using the pressure pulse wave detector that is expensive and difficult or cumbersome to handle.
It is noted that, in a first portion of the pulse of the cuff pulse wave that precedes the maximum point of the pulse, the amount of difference between the waveform of the cuff pulse wave and the waveform of the pressure pulse wave slightly differs from that in a second portion of the pulse of the cuff pulse wave that follows the maximum point of the pulse. In view of this, preferably, the pseudo-pressure-pulse-wave determining means determines a first portion of the pseudo pressure pulse wave by correcting the respective pressure values of the respective points of the first portion of the heartbeat-synchronous pulse that precedes the maximum point of the pulse, according to a first correction relationship, and determines a second portion of the pseudo pressure pulse wave by correcting the respective pressure values of the respective points of the second portion of the pulse that follows the maximum point of the pulse, according to a second correction relationship differing from the first correction relationship. According to this arrangement, the pseudo pressure pulse wave determined by the pseudo-pressure-pulse-wave determining means becomes more similar to a true pressure pulse wave, so that a more accurate diagnosis can be made by using the determined pseudo pressure pulse wave.
Preferably, the pseudo-pressure-pulse-wave determining means selects, from a plurality of correction relationships corresponding to a plurality of pulse-pressure ranges, one correction relationship based on a pulse pressure determined based on the blood pressure value determined by the apparatus, and determines the pseudo pressure pulse wave by correcting the respective pressure values of the respective points of the heartbeat-synchronous pulse, according to said one correction relationship. According to this arrangement, the correction relationship used where the pulse pressure is small differs from that used where the pulse pressure is great, so that the determined pseudo pressure pulse wave becomes more similar to a true pressure pulse wave.